This invention relates generally to the fabrication of needles. More particularly, this invention relates to a technique of forming vertical, hollow needles within a semiconductor substrate.
Traditionally, needles have been formed with stainless steel. Recently, techniques for forming needles from semiconductors have been disclosed. Several techniques for fabricating needles within a semiconductor substrate have formed the needles in the horizontal plane of the semiconductor substrate. For example, the technique of U.S. Pat. No. 5,928,207, invented by Albert P. Pisano and Kyle S. Lebouitz, and entitled xe2x80x9cMicroneedle with Isotropically Etched Tip, and Method of Fabricating such a Devicexe2x80x9d, uses isotropic etching to form microneedles in the horizontal plane of a semiconductor substrate. This patent, which is assigned to the assignee of the present application, is incorporated by reference herein.
Another semiconductor fabrication technique for forming needles is disclosed by Neil H. Talbot, Christopher G. Keller, and Albert P. Pisano, in their U.S. patent application Ser. No. 09/044,398, filed Mar. 18, 1998, entitled xe2x80x9cApparatus and Method for Fabricating Needles Via Conformal Deposition in Two-Piece Moldsxe2x80x9d. This technology forms a needle via conformal deposition within a horizontally-oriented chamber defined by a two-piece mold.
Kyle S. Lebouitz and Albert P. Pisano also developed a matrix of isotropically etched tips forming an xe2x80x9cabraderxe2x80x9d used to abrade epidermis and thereby facilitate transdermal drug delivery. This technology is described in U.S. patent application Ser. No. 09/106,991, filed Jun. 29, 1998, entitled xe2x80x9cTransdermal Probe with an Isotropically Etched Tip, and Method of Fabricating such a Devicexe2x80x9d. This patent application, which is assigned to the assignee of the present application, is also incorporated by reference herein. The structure disclosed in the patent application is formed using only isotropic etching. The structure does not have through holes. Further, the structure is not cut into individual needles, as the matrix is used as a single structure forming an abrader.
A related transdermal drug delivery device is disclosed by S. Henry, et al., in xe2x80x9cMicrofabricated Microneedles: A Novel Approach to Transdermal Drug Delivery,xe2x80x9d Journal of Pharmaceutical Sciences, Vol. 87, No. 8, pp. 922-925, 1998. This publication discloses an array of solid needles that may be used for transdermal drug delivery. An improvement upon this technology is described by several of the same authors in V. D. McAlister, et al., xe2x80x9cThree-Dimensional Hollow Microneedle and Microtube Arrays,xe2x80x9d Trandsducers ""99 Conference Proceedings, pp. 1098-1101, Sendai, Japan, 1999. The array of needles described in this publication have interior channels. The authors describe how the disclosed arrays can be used for drug delivery or controlled micro-combustion applications.
Despite recent advances in the fabrication of semiconductor-based needles, there still exits a need for improved needle formation techniques. For example, it would be desirable to provide a technique for forming needle walls with various vertical slopes. It would also be desirable to provide a technique for forming needle tips with a variety of shapes.
A method of forming a needle includes the step of anisotropically etching a channel into the back side of a semiconductor substrate. The front side of the semiconductor substrate is then isotropically etched to form a vertical axial surface surrounding the channel. The resultant needle has an elongated body formed of a semiconductor material. The elongated body includes an axial surface positioned between a first end and a second end. The axial surface defines a channel between the first end and the second end.
The invention provides a variety of techniques for altering the shape of the fabricated needles. For example, a combination of anisotropic and isotropic etching is used to produce needles with steep vertical walls, while isotropic etching alone is used to produce needles with sloping vertical walls that terminate in wide bases. The wide based embodiments operate to withstand relatively large lateral forces. The mask displacement technique of the invention can be used to form a sloping tip with a single circumferential termination point.